Idiosyncratic adverse drug reactions are unpredictable, dose independent and potentially life threatening; this makes them a major factor contributing to the cost and uncertainty of drug development. Clinical data suggests that many such reactions involve immune mechanisms, and genetic association studies have identified strong linkage between drug hypersensitivity reactions to several drugs and specific HLA alleles. One of the strongest such genetic associations has been found for the antiviral drug abacavir, which causes severe adverse reactions exclusively in patients expressing the HLA molecular variant B*57:01. The mechanism by which abacavir induces this pathologic T cell response remains unclear. We have strong preliminary data that systemic adverse drug reactions can be caused by the formation of an altered peptide repertoire that triggers T cell immunity in HLA associated drug hypersensitivity. Using in vitro binding assays, MHC ligand elution and X- ray crystallography, we have shown that abacavir binds inside the F-pocket of HLA B*57:01, thereby altering its specificity. This supports a novel hypothesis that systemic adverse drug reactions are due to the formation of an altered peptide repertoire that triggers T cell immunity in HLA associated drug hypersensitivity. The objective is to utilize this approach to define the roles of three drugs in altering peptide binding to HLA and to guide development of drug variants that have a reduced likelihood of causing HLA linked drug hypersensitivity. Our specific aims are: 1) Identify peptides that bind to HLA-B molecules in a drug dependent manner. We will perform affinity measurements of a combinatorial peptide library in the presence or absence of the drug to the relevant HLA-B molecule and confirm drug effects detected in the MHC binding assays using MHC ligand elution experiments with live cells. 2) Determine structures of drugs complexed to HLA-B molecules associated with hypersensitivity. We will determine the structure of drug/peptide/HLA complexes by X-ray crystallography. As available, we will utilize the peptides identified in Aim 1 for crystallization, which we have foun to facilitate drug/peptide/HLA complex formation in the case of abacavir. 3) a. Characterization of functional consequences of the drug-induced altered peptide repertoire. b. Characterization of the functional effects of drug variation on HLA associated drug recognition. We will utilize the structural information from Aim 2 to design drug variants that do not bind HLA molecules and are therefore predicted to not cause T cell immune responses. We will test the drug variants for T recognition using samples from hypersensitive patients. The ability to test specific drug HLA combinations in molecular binding assays for effects on the self repertoire could drastically improve the ability to detect HLA linked drug hypersensitivities. Moreover, the drug variation approach may be attractive to global healthcare endeavors where a potentially small change in a proven therapy would simplify treatment.